I am not sure what exactly you are doing, but here is some general info about sharpness:
a) The DA21mm isn't the sharpest lens in the world. On the other hand, macro lenses are often spectacularly sharp. This means maybe you are used to sharper lenses, but that doesn't mean the DA21mm is bad. I have the DFA 100mm macro and it beats pretty much all my lenses in terms of sharpness and amount of detail rendered. But on the other hand, it doesn't have as much character as some other lenses. Sharpness is not everything
b) To get optimal sharpness you have to use a sturdy tripod and 2 sec timer, low ISO (so the photo isn't fuzzy) and an aperture value that gives you the DoF you need.
c) Environmental effects can make things look unsharp. Atmosphere might not be clean, there might be a breeze moving the leaves and grass, wind shaking the tripod, etc.
d) Post processing is important. If you look at the top notch "pro" photos, they all sharpen their photos a lot in post. Some just shoot raw and use the Sharpness slider, others use plugins and addons, some even use complicated techniques. If you shoot raw, you need to sharpen it up correctly. Be careful with noise reduction, as some NR can cause details to vanish, which makes the photo look soft like a painting.
e) AF or MF? The problem is that many modern lenses do not have a correctly labelled infinity. Some lenses allow focus past infinity, others have no labels at all, and some have a label, but its wrong. This makes MF difficult and confusing. AF might not be very useful when it comes to near infinity subjects, because you often want to zone focus rather than just dial to infinity. You can try using Live view contrast-detect AF btw. For better focus, you can either get the lens and camera calibrated (together) or learn how to focus to get what you want.
Reason why so many lenses have a poor focusing ring these days is that they have a short focus throw for faster AF, and they have space for "error" (so they go past infinity. This allows lower QC standards and allows the lens to be used in many different temperatures, since materials expand). For the average user, this is not a problem. If you seek perfection, it might be an obstacle.
Beyond that, I don't think your examples are unsharp or bad. But there is a learning curve with wide angle lenses and processing them correctly. Also, the DA 12-24mm is probably sharper than the DA 21mm. I think the DA 21mm was mostly meant for street photography and group photos, and the DA 14mm and DA 15mm were meant for landscapes, but I don't actually have experience with them DA 21mm, so I can't give you more specific tips. Ive seen some great landscape photos posted to this forum taken with the DA 21mm. Feel free to check the sample photo gallery.